Magnetic spin switching in confined magnetic structures has applications for example in memory, logic and sensor device, in Spin-LEDs etc. Switching of a magnetic element is conventionally achieved by applying external magnetic fields, for example using a coil setup or local striplines. This has a number of drawbacks, since the field-generating striplines are cumbersome to fabricate and the currents used to create the magnetic field can lead to heating. Moreover, for noise reduction the striplines are often electrically insulated from the structures, so that separate lines have to be used for read-out and field generation, which increases the complexity of the system.
Typical novel non-volatile memory devices such as MRAM are cross-point memories. A memory cell is addressed and switched by applying a current pulse in the two orthogonal striplines (the word line and the bit line), which cross at the position of the addressed cell. The other cells that are positioned along these two striplines will feel the field generated by one stripline, i.e., typically 70% of the field present at the intended cell at which both current-carrying lines intersect. Thus the switching fields of the cells have to be extremely uniform and reproducible in order to obtain switching of only the addressed cell, and the operating window for the switching fields can be small. In other words, switching has to be reliably generated by two intersecting striplines but must not be generated by a single stripline. Proposals to overcome this obstacle have been made, a prominent one being Motorola's so-called “toggle switching”. This approach, however, has the drawback of reducing the maximum speed of the device and requires at least one additional line per memory cell, thus compromising the ultimate integration density. In all Magnetoresistive Random Access Memory (MRAM) approaches, the switching fields of the cells increase as the lateral size is reduced, at least as long as the element sizes are large enough such that the superparamagnetic limit is not relevant. At the superparamagnetic limit the element size is so small that the energy differences between the magnetization states are not far above kT, where k is Boltzmann's constant and T is the temperature, and hence the temperature tends to destabilize the magnetic state. This requires higher currents in the striplines to switch the elements as the element size goes down. This effect in turn increases the unwanted effect of element heating and also of electromigration in the structure. Further, in general, with increased force of the switching field the switching field distribution of all elements increases as well.
An alternative approach is to use a multilayer pillar consisting of a Giant Magnetoresistance (GMR) stack (such as a Co/Cu/Co trilayer with different Co thicknesses) and switch the magnetically softer ferromagnetic layer by high current densities flowing across the stack. It has been found that electrons flowing from the magnetically hard to the magnetically soft layer align the layers parallel, and opposite currents align them antiparallel. While this simplifies the fabrication process, as read-out and writing are done using the same current path and consequently the same electrodes may be used, only GMR stacks can be used. Tunnelling Magnetoresistance (TMR) stacks, which exhibit a magnetoresistance effect based on a similar physical principle as GMR, may so far not be used, since their resistance is too high to obtain the current densities necessary for switching by spin-torque. However, due to the large energy needed for read-out and writing, GMR stacks are nowadays not considered for memory elements anymore, but rather the high-resistance TMR elements, such as NiFe/Al2O3/Co stacks.
Currently,-induced domain wall propagation is a known effect described for example in L. Berger, J. Appl. Phys. 55, 1954(1984), in M. Kläui et al., Appl. Phys. Lett. 83, 105(2003), or A. Yamaguchi. Phys. Rev. Lett. 92, 077205(2004).